1. Field of the Invention
The present invention relates to satellite communication systems and, more particularly, to an interface which enables, among other things, interactive services in a communication system such as a satellite television system.
2. Background Information
The use of satellites to distribute television signals is known in the broadcasting industry and has helped revolutionize television distribution systems. The first generation of satellite television systems employed communication satellites that encircle the earth in so-called “geosynchronous” orbits, meaning that the satellites encircling the earth are stationary relative to fixed points on the earth. A geosynchronous satellite needs to maintain a high altitude in order to remain in geosynchronous orbit. One aspect of a geosynchronous orbit is that it enables a single satellite to distribute television signals to entire continents, large portions of continents, and/or large areas on the earth.
The next generation of satellite television systems involves the use of low earth orbit (“LEO”) satellites and/or medium earth orbit (“MEO”) satellites. As the names imply, these satellites occupy much lower orbits than geosynchronous satellites. An LEO and/or MEO satellite system is conducive for providing various interactive services such as interactive television services, internet services (e.g., electronic mail, web surfing, etc.) and the like, since the round trip signal propagation time is substantially less than in a geosynchronous satellite system. However, because of their lower orbits, multiple LEO/MEO satellites are required in order to distribute signals to the same geographical area as that of a geosynchronous satellite.
With LEO/MEO satellite systems omnidirectional antennas can be used, providing simple, spatially inefficient links that do not need to track satellites. Omnidirectional systems provide very low bandwidth due to the extremely low gain of the omnidirectional antenna in the link. To achieve high bandwidth and space division multiplex, a high gain antenna must be used and the satellites tracked. To therefore provide a high bandwidth continuous data link to non-geosynchronous satellites, the ability to track two satellites is required, since as one satellite passes over the horizon, a link to another satellite must have been established and operating to take over the link. By tracking two satellites, a make-before-break switch can be made between the satellites.
In the context of satellite television systems, the use of multiple non-geosynchronous satellites (i.e. LEO and MEO satellite systems) requires that a subscriber's television equipment be capable of tracking and receiving signals from multiple satellites. Accordingly, a subscriber's television equipment should include multiple receivers that operate in a simultaneous manner. Moreover, in order to accommodate interactive television services, it is also desirable for a subscriber's television equipment to include a means for transmitting signals to the satellites. In the case of the next generation television satellite system, the consumer's television equipment must therefore have a ground station that is capable of receiving two satellite signals and provide an uplink signal to a satellite. The ground station typically consists of an indoor unit and an outdoor unit.
Since the proposed ground station in the next generation television satellite system is a consumer apparatus, it is desirable to minimize the cost of the consumer ground station. One major cost in a consumer ground station is the connection from the indoor unit to the outdoor unit. In such a make-before-break system, there is a need to demodulate two satellite signals. Using a traditional LNB (Low Noise Block) downconverter, this implies carrying two wideband signals (up to ≈1 GHz each) on a single piece of cable. In addition, the uplink signal also needs to be carried on the same single cable.
In U.S. patent application Ser. No. 10/029,645, entitled “Bi-Directional Communication Apparatus” by Ramaswasmy et al. filed Dec. 21, 2001, a satellite television ground station is shown that is an example of a satellite television ground station that receives first and second satellite television signals. The first and second satellite television signals are down-converted in an outdoor unit of the satellite television ground station and transmitted to an indoor unit of the satellite ground station. A signal transmitting module receives an RF signal from the indoor unit and up-converts the RF signal for selectively providing the up-converted signal in response to a selection signal. A reference frequency from the indoor unit is provided to a reference oscillator generator in the outdoor unit. The reference oscillator generator provides an oscillator signal to various components of the outdoor unit. The satellite television ground station, however, is only elementary in design and does not address the problems solved in the present invention.
What is therefore needed is a satellite system that, in one instance, overcomes the above and other current shortcomings and/or drawbacks of current satellite systems. What is therefore further needed is a ground-based satellite television system that, in one instance, overcomes the above and other shortcomings and/or drawbacks in current ground-based satellite television systems.